At the present time, the nitrogen oxide and particle emissions (dust) of a diesel motor optimized for power and consumption can only be insignificantly reduced on the basis of combustion-oriented measures. In order to be able to achieve the future exhaust limit values prescribed by legislators, an after-treatment is required of the exhaust gasses of diesel motors.
A noticeable reduction in the NO.sub.x emission of a diesel motor can be achieved by applying what is referred to as Selective Catalytic Reduction. In the SCR method, gaseous ammonia NH.sub.3, ammonia in aqueous solution or urea is injected into the exhaust gas system as a reduction agent, so that the chemical reactions EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O EQU 2NO.sub.2 +4NH.sub.3 +O.sub.2 .fwdarw.3N.sub.2 +6H.sub.2 O
can occur at a catalyst. Approximately 0.9-1.1 mol NH.sub.3 are required for reducing one mol NO.sub.x in the exhaust gas of the diesel motor. When less ammonia NH.sub.3 is injected, the catalyst no longer works with the highest efficiency. An over-dosing is likewise to be avoided since unused ammonia NH.sub.3 is emitted into the atmosphere.
The SCR method disclosed by German reference DE 36 10 364 can reduce the NO.sub.x part in the exhaust gas of firing systems by more than 80% and can simultaneously limit the NH.sub.3 emission to less than 5 ppm. The dosing of the reduction agent is monitored by a computer that evaluates the output signal of a NH.sub.3 sensor located in the exhaust system following the NO.sub.x converter and readjusts the delivery of reduction agent by driving a conveying unit as warranted. An electrochemical cell, that contains a cup-shaped body fabricated of stabilized zirconium dioxide as a critical component, serves as the NH.sub.3 sensor. Two electrodes are applied to the solid electrolyte, whereby the outer electrode exposed to the exhaust gas is composed, for example, of TiO.sub.2, PtV.sub.2 O.sub.5 or V.sub.2 O.sub.5 and the inner electrode exposed to a reference gas (air) is composed of platinum.
The method disclosed by German reference DE 36 06 635 for reducing the NO.sub.x concentration utilizes the high temperatures of the exhaust gas in order to evaporate the reduction agent present in solid or liquid form in order to split it into reactive components. Thereafter, the reactive components are injected via a bundle of pipelines into a comparatively cool zone of the exhaust system where they react with the nitrogen oxides to form nitrogen, water and carbon dioxide. A sensor working according to a chemiluminescence comparison method measures the NO.sub.x concentration. The output signal thereof is supplied to a regulator having a prescribed rated value that drives the motor of a dosing valve.